1. Field of the Invention
The present invention relates to an ignition device for an internal combustion engine, and in particular, to an ignition device used as a multiple electric discharge ignition type ignition device that causes an ignition plug to perform multiple times of ignition electric discharge in one combustion cycle.
2. Description of Related Art
In a spark ignition type internal combustion engine, ignition electric discharge is caused in an ignition plug in a combustion chamber by driving an ignition device consisting of an ignition coil and the like. Fuel introduced into the combustion chamber is combusted by the ignition electric discharge. In recent years, in order to improve a combustion state in the combustion chamber or to reduce power consumption, a multiple electric discharge ignition type ignition device has been proposed (for example, as described in Japanese Patent Gazette No. 2811781). The multiple electric discharge ignition type ignition device causes the ignition plug to perform multiple times of ignition electric discharge during one combustion cycle.
An example of the multiple electric discharge ignition type ignition device is shown in FIG. 14 as a circuit diagram. For example, the ignition device is used as an ignition device of an in-vehicle internal combustion engine. Next, an explanation will be given by assuming that the internal combustion engine is a four-cylinder engine that performs the ignition sequentially in the cylinders #1, #3, #4, and #2 in this order.
As shown in FIG. 14, the ignition device is mainly composed of an ECU (electronic control unit) and a drive circuit (i.e., an igniter). The ECU is mainly constituted by a well-known microcomputer. The ECU acquires engine operation states such as engine rotation speed and an accelerator operation amount (i.e., a pressing amount of an accelerator by a driver) and inputs the operation states to a signal output section 101. The signal output section 101 calculates optimum ignition timing (equivalent to ignition timing in normal control) based on the engine operation states and generates ignition signals IGt1, IGt2, IGt3, IGt4 (signals corresponding to the respective cylinders #1, #3, #4, #2) corresponding to the optimum ignition timing. The signal output section 101 calculates a multiple electric discharge period (i.e., a period for continuously performing multiple electric discharge ignition) based on the engine operation states and generates a multiple period signal IGw corresponding to the multiple electric discharge period. The drive circuit has a control section 201 as a circuit for performing predetermined control related to the ignition in the internal combustion engine. The control section 201 has an energy accumulation control section 201a and a multiple ignition control section 201b. The control section 201 controls energization/de-energization of an ignition coil L1 by the respective control sections based on the ignition signals IGt1, IGt2, IGt3, IGt4 and the multiple period signal IGw from the signal output section 101.
In more detail, in the drive circuit, an in-vehicle battery B1 (a direct-current power supply), a coil L10 for accumulating an energy, and a transistor Tr11 as a switching element are connected in series with each other. A series circuit of a diode D1 for preventing a backflow of current by rectification, a primary ignition coil L1a, and a normally-off type transistor Tr21 (an end thereof is grounded), which is switched on when a logically high (H) electric potential is applied to a gate thereof, is electrically connected in series with the coil L10 (and in parallel with the transistor Tr11). A capacitor C1 (an end thereof is grounded) is electrically connected in series with the diode D1 (and in parallel with the primary ignition coil L1a).
The primary ignition coil L1a is paired with a secondary ignition coil L1b to constitute the ignition coil L1. The ignition coil L1 is provided for each cylinder of the engine (the internal combustion engine) In the case of the four-cylinder engine, four ignition coils L1 are provided. In the ignition device, with such the ignition coil L1, high voltage is induced in the secondary ignition coil L1b by using the electromagnetic induction by the primary ignition coil L1a. Thus, the high voltage and eventual ignition electric discharge are caused in an ignition plug (i.e., an ignition plug provided to the combustion chamber of the engine) connected to the coil L1b. 
FIG. 15 shows details of the structure of the multiple ignition control section 201b. As shown in FIG. 15, in the multiple ignition control section 201b, an AND circuit 211 and a cylinder determination circuit 221 are provided for the cylinder #1. A drive signal Dr2 and a cylinder determination signal G11 are inputted into the AND circuit 211. An output of the AND circuit 211 is inputted into the gate of the transistor Tr21 as a switching signal G21. The drive signal Dr2 is an output of an AND circuit 210, into which the multiple period signal IGw and a drive signal Dr1 are inputted. The drive signal Dr1 is a pulse signal that repeatedly alternates between ON and OFF in a predetermined cycle during a period from falling of one of the ignition signals IGt1, IGt2, IGt3, IGt4 to falling of the multiple period signal IGw. The cylinder determination signal G11 indicates the cylinder as an ignition target and is generated by the cylinder determination circuit 221 based on the ignition signal IGt1. In more detail, the cylinder determination signal G11 becomes logically high (H) since the ignition signal IGt1 falls until a specified time elapses after the falling of the ignition signal IGt1.
An AND circuit 212 and a cylinder determination circuit 222 similar to the AND circuit 211 and the cylinder determination circuit 221 are provided also for the cylinder #3. The drive signal Dr2 and a cylinder determination signal G12 are inputted into the AND circuit 212. An output of the AND circuit 212 is inputted into a gate of a transistor Tr22 as a switching signal G22. Although not shown in the drawing, the ignition device has similar AND circuits and cylinder determination circuits also for the cylinders #2, #4.
The drive circuit shown in FIG. 14 having the multiple ignition control section 201b energizes the primary ignition coil L1a by discharging the electric charge accumulated in the capacitor C1 to cause the ignition plug to perform the first ignition electric discharge at the timing based on the ignition signal IGt1 and the multiple period signal IGw from the signal output section 101. Then, the drive circuit alternately switches on and off the transistor Tr11 and the transistor Tr21 in a fixed cycle. Thus, the current flows through the coil L10 and eventually an inductive energy (electric energy) is accumulated in the coil L10 when the transistor Tr21 is switched off and the transistor Tr11 is switched on. The inductive energy accumulated in the coil L10 is discharged and eventually the current flows through the primary ignition coil L1a when the transistor Tr11 is switched off and the transistor Tr21 is switched on. The ignition device thus causes the electricity to flow through the secondary ignition coil L1b in a forward direction and a backward direction during the multiple electric discharge period such that the ignition coil L1 repeatedly performs the ignition electric discharge. Thus, the ignition device performs the multiple electric discharge with the ignition plug electrically connected to the ignition coil L1 (in more detail, to the secondary ignition coil L1b).
FIG. 16 is a timing chart showing an operation mode of the above-described ignition device. Next, an operation mode of the above-described ignition device will be explained by specifically paying an attention to an ignition in a cylinder (cylinder #1) among the multiple cylinders. Fundamentally, the ignition control is performed by the similar operation also for the other cylinders. In FIG. 16, part (a) shows a transition of the ignition signal IGt1, part (b) shows a transition of the multiple period signal IGw, part (c) shows the switching signal G21 for the transistor Tr21, and parts (d) to (g) respectively show currents i11 i12, i1, i2 respectively flowing through the coil L10, the transistor Tr11, the primary ignition coil L1a and the secondary ignition coil L1b. 
As shown in part (a) of FIG. 16, in this example, the ignition signal IGt1 is switched on (i.e., becomes logically high (H)) at timing t101. Thus, the transistor Tr11 is switched on, and the currents i11, i12 increase gradually as shown in parts (d) and (e) of FIG. 16.
Then, the ignition signal IGt1 is returned from the ON state to the OFF state (logically low (L) state) at timing t102. As shown in part (b) of FIG. 16, the multiple period signal IGw is switched on in synchronization with the falling of the ignition signal IGt1. In a period from t102 to t109 in which the multiple period signal IGw is ON (i.e., a period from the falling of the ignition signal IGt1 to the falling of the multiple period signal IGw), the switching signal G21 for the transistor Tr21 turns into a pulse signal that repeatedly alternates between ON and OFF in a predetermined cycle as shown in part (c) of FIG. 16. Thus, the transistor Tr21 is switched on and off, and correspondingly, the currents i11, i12 start to repeat increasing and decreasing as shown in parts (d) and (e) of FIG. 16.
In more detail, the transistor Tr11 is switched off at the timing t102. Thus, the electric energy accumulated in the coil L10 and the capacitor C1 is discharged to the primary ignition coil L1a and eventually to the secondary ignition coil L1b, so the electric discharge is performed by the ignition plug electrically connected to the coil L1b. In order to effectively exploit the energy accumulated in the capacitor C1, it is preferable to set the first pulse width (a period from t102 to t103) to be wider than the other following pulse widths (periods t103-t104, t104-t105, etc.).
Furthermore, at timing t103 when a predetermined time elapses after the timing t102, the transistor Tr21 is switched off and the transistor Tr11 is switched on, so the energy is accumulated in the coil L10. Then, the transistor Tr11 is switched off and the transistor Tr21 is switched on at following timing t104. Thus, the electric energy accumulated in the coil L10 is discharged to the primary ignition coil L1a and eventually to the secondary ignition coil L1b, so the electric discharge is performed by the ignition plug as described above. Also after that, the accumulation of the energy and the discharge of the energy are performed alternately at respective timings t105-t109. Thus, multiple electric discharge ignition is realized. The multiple period signal IGw is switched off and the ON/OFF drive of the switching signal G21 stops at the following timing t109.
Next, another example of the multiple electric discharge ignition type ignition device will be explained with reference to FIG. 17. FIG. 17 is a circuit diagram corresponding to FIG. 14. The same sign is used for the same element shown in both of FIGS. 14 and 17 and the explanation of the element is not repeated here. The ignition device shown in FIG. 17 is also used as an ignition device for an in-vehicle internal combustion engine.
The ignition device shown in FIG. 17 is also mainly composed of an ECU (electronic control unit) and a drive circuit (i.e., an igniter) like the device shown in FIG. 14. The ignition device shown in FIG. 17 has a signal output section 102, a control section 202, an energy accumulation control section 202a, and a multiple ignition control section 202b as sections similar to the signal output section 101, the control section 201, the energy accumulation control section 201a, and the multiple ignition control section 201b of the device shown in FIG. 14. Among the sections, the multiple ignition control section 202b has a structure similar to that of the multiple ignition control section 201b shown in FIG. 15. However, in place of the transistor Tr11 and the coil L10 of the device shown in FIG. 14, the device shown in FIG. 17 has a Vdc booster mechanism 302a and a Vcdi booster mechanism 302b to realize more precise ignition control.
The Vdc booster mechanism 302a has a capacitor of a large capacity and charges the capacitor at prescribed voltage. If the transistor Tr21 in the ignition device is switched on, high voltage is generated in the secondary ignition coil L1b and therefore excitation voltage is generated in the primary ignition coil L1a. The current i1 flowing through the primary ignition coil L1a can be maintained for a certain time by charging the capacitor of the Vdc booster mechanism 302a at voltage (for example, 50V) higher than the excitation voltage. The Vdc booster mechanism 302a is connected to the capacitor C1 through the diode D1. Thus, the backflow of the current from the capacitor C1 to the Vdc booster mechanism 302a is prevented.
The Vcdi booster mechanism 302b has a function to charge the capacitor C1. With this structure, re-ignition can be performed by charging the capacitor C1 with the Vcdi booster mechanism 302b until contact voltage between the capacitor C1 and the primary ignition coil L1a reaches a voltage (for example, 250V) higher than a voltage calculated by multiplying a turn ratio of the ignition coil L1 and re-ignition request voltage, and by discharging the capacitor C1 after the charging. The turn ratio of the ignition coil L1 is calculated by dividing the number of turns of the primary ignition coil L1a by the number of turns of the secondary ignition coil L1b. 
FIG. 18 is a timing chart showing an operation mode of the ignition device described above, In FIG. 18, part (a) shows a transition of the ignition signal IGt1, pad (b) shows a transition of the multiple period signal IGw, part (c) shows a transition of the switching signal G21 for the transistor Tr21 part (d) shows a transition of a quantity VC1 of the electric charge accumulated in the capacitor C1, part (e) shows a transition of the current i1 flowing through the primary ignition coil L1a, and part (f) shows a transition of the current i2 flowing through the secondary ignition coil Lib respectively.
As shown in part (a) of FIG. 18, in this example, the ignition signal IGt1 is switched on (i.e., becomes logically high (H)) at timing t201 and is switched off (i.e., becomes logically low (L)) at timing t202. In a period from t202 to t209 corresponding to the period from t102 to t109 shown in FIG. 16, the switching signal G21 turns into a pulse signal that repeatedly alternates between ON and OFF in a predetermined cycle. The ignition device of this example switches the transistor Tr21 between ON and OFF based on the current i2 and the electric charge quantity VC1 accumulated in the capacitor C1 as shown in parts (c), (d) and (f) of FIG. 18. In more detail, the transistor Tr21 is switched off when the current i2 reaches a predetermined threshold (for example, 50 mA), and the transistor Tr21 is switched on when the electric charge quantity VC1 reaches another predetermined threshold (for example, 250V). The threshold (for example, 50 mA) of the current i2 is set at a value that does not cause a misfire of the combustion in the combustion chamber.
Thus, each of the above-described ignition devices performs the ignition control of each cylinder based on the falling of each of the ignition signals IGt1, IGt2, IGt3, IGt4. However, as for the various signals related to the ignition control, there is a possibility that the ignition signal IGt1 is outputted for an abnormally long time, for example, as shown in FIG. 19, due to an engine stall, disturbances (noises) and the like. In such the case, there is a concern that the above-described ignition device erroneously recognizes (misidentifies) the timing t202, at which the ignition signal IGt1 falls, as the ignition timing and performs the ignition at the timing different from desired timing t202a. in such the case, there is a concern that a backfire is caused with an opening operation of an engine intake valve because of delay in the ignition timing, causing a large damage to the engine or peripheral devices such as injectors and sensors.